Summary At 2020 Pacific daylight time, the pilot of the Eurocopter AS350B2 (Astar) helicopter (C-GSHH, serial number3192) landed on a recently prepared mountainside helipad, five nautical miles west of the extinct Flourmill Volcano, at 5200feet elevation. With the helicopter still running at flying rotor rpm and light on the skids, four passengers boarded with a small amount of personal equipment and prepared for take-off. The pilot increased collective pitch to bring the helicopter into the hover, but the engine parameters were approaching their limits, and he discontinued the take-off and lowered the collective. The left rear passenger got out, and the pilot again raised the collective, lifting the helicopter into a stable five-foot hover over the pad. Satisfied this time with the engine readings, the pilot increased collective pitch and climbed to approximately 20feet while purposely allowing the nose to swing to the left to turn downhill for the transition into forward flight. As the helicopter turned through 100degrees of left turn, the low rotor rpm warning horn sounded and the pilot decided to return to the pad. He allowed the left turn to continue but, by the time the helicopter returned to the original heading, it had drifted approximately 20feet downhill from the pad and was still descending. The main rotor blades then struck a large tree stump adjacent to the pad and the helicopter rolled over, coming to rest on its left side, almost inverted. The three passengers quickly escaped from the helicopter, but the pilot delayed his exit to shut down the engine, which had continued to run. After he had secured the engine, fuel valve, and electrical switches, the pilot exited the cockpit. The four occupants received minor injuries, and the helicopter was substantially damaged. The emergency locator transmitter activated automatically at rollover. There was no fire. Ce rapport est galement disponible en franais. Other Factual Information History of Flight The accident helicopter was on contract to the British Columbia Forestry Service (BCFS) to transport firefighters and their equipment to various sites in the Williams Lake vicinity. The accident flight was to relocate 11firefighters and light personal equipment from the mountainside pad for Fire154 (BCFS fire number154) to a staging area at the base of the mountain, a distance of approximately one nautical mile. Pilot The pilot was trained and licensed appropriately for the AS350B2 helicopter and the firefighting support mission. He was an experienced and qualified helicopter pilot and had worked for the operator in forest fire-management for several years. He had accumulated over 3000hours of flight time, including over 1000hours in the AS350model helicopter. A review of his flight and duty times for the period leading up to the accident revealed no deviation from Transport Canada (TC) regulations respecting flight and duty time limits, nor had he recently engaged in arduous operational activities. Weather No formal weather observation exists for the accident site, but weather reports in adjacent areas and reports from the witnesses at the site show that the general weather conditions were suitable for VFR (visual flight rules) flight. There was a light wind from the northeast on the surface and a moderate wind at the tree-top level, with a significant down-flowing wind over the accident site. The outside air temperature in the hours leading to the time of the accident was in the order of 24C to 28C. However, it had cooled following recent rain showers, and the helicopter's outside air temperature gauge was reading 22C to 24C. For the performance calculations included in this report, a temperature of 20C was used. Aircraft Maintenance A review of all the aircraft technical logs and maintenance records indicates that C-GSHH was certificated and maintained in accordance with existing regulations and standards. No maintenance deficiencies were found, nor were there any deferred mechanical defects. This helicopter was manufactured in 1999by Eurocopter in Marignane, France. At the time of the accident, the helicopter and its engine had accumulated approximately 1880 hours total flight time since new. Dual controls were not fitted to the helicopter, and the throttle control quadrant on the cockpit floor was protected from passenger interference by an approved throttle guard. Fuel The pilot had refuelled the helicopter approximately 40minutes before landing at the Fire154 helipad. The pilot estimated that approximately 68percent (650pounds) of total fuel remained at the time of the accident. Investigators removed approximately 365litres of fuel from the helicopter fuel tank after the accident, confirming that there were approximately 650pounds of fuel on board at the time of the accident. Fuel samples taken from the engine and from the airframe fuel systems and filters were examined for contamination; none was found. Accordingly, fuel supply and quality are not considered to have contributed to this accident. Aircraft Weight and Balance The maximum certificated internal gross weight for this helicopter is 4961pounds, and the limits of the centre of gravity (CG) range from 124.8to 137.8inches from the datum, depending on the helicopter's gross weight. The helicopter weighed approximately 4825pounds at the time of the accident, with a CG of approximately 126.1inches from the datum. At that weight, the CG was slightly less than the maximum permitted forward CG limit of 125.9inches. Helicopter Performance Helicopter performance is predicated on altitude, temperature, and gross weight. The theoretical performance limits are readily available to pilots in Section5, Regulatory Performance Data, of the Direction Gnrale de l'Aviation Civile-approved rotorcraft flight manual (RFM). In part, the performance curves in the RFM predict the weight that a helicopter should be able to carry, given certain in-flight conditions, as well as identify the maximum certificated weight for those same conditions. Specifically, the RFM contains two performance charts that accurately determine the maximum weight that the helicopter can hover in, or out of, ground effect, namely HIGE1 (hover in-ground-effect) and HOGE2 (hover out-of-ground-effect) performance charts. However, since the helicopter reached a height of 15to 20feet at take-off, and because the slope of the accident site terrain is quite steep, it is unlikely that any benefit from ground effect was gained during the events immediately leading up to this accident. Accordingly, the HOGE chart is the appropriate reference to determine the ability of the helicopter to take off from the accident helipad and hover successfully. Using the HOGE curves (seeAppendixA), assuming an outside air temperature of 20C, a pressure altitude of 5200feet, and a gross weight of 4825pounds, at the second take-off attempt the helicopter was approximately 150pounds less than the maximum weight permitted for those conditions. As ambient air temperature increased to 25C, this margin would have been progressively eroded to zero. The actual performance of the helicopter would also have been affected by several other factors, including wind and wind gusts, pilot handling techniques, and engine and rotor system efficiency. Calculations show that the vertical climb speed would have been in the order of 300feet per minute for take-off engine power, and 590feet per minute for maximum available power. Each would have been increased by 230feet per minute as a result of left pedal input during the climb. Accordingly, the maximum possible rate of climb would have been 820feet per minute in ideal conditions. Engine Examination and Performance This Astar helicopter was equipped at manufacture with the Turbomeca Arriel1D1 turboshaft engine, serial number9631. Following the accident, the engine was removed from the helicopter, examined, and run in a TC-approved test cell. The results of the examinations and test runs revealed no deficiency or degradation in performance that would have contributed to the loss of rotor rpm. The tests confirmed that the engine met the manufacturer's specifications for power delivery, with the following exception. By calculation, the minimum power before engine repair is required3 is 523kW for ambient temperatures of 25C and below. During the test for maximum power, the assessed value was 5kW4 below the repair limit of 523kW, that is, the equivalent of approximately one per cent of the test-cell target value. This performance differential is not uncommon for engines with similar accumulation of flight time since new - in the order of 1900hours - and is an anticipated and gradual deterioration. Since it represents a difference of available torque of approximately one per cent, it was considered negligible. With the engine installed in the helicopter, this differential in available power would have been overcome by the pilot applying collective pitch to increase the power demand up to and, if necessary, exceeding the certificated engine operating limits. The power differential seen in the test cell would not have precipitated the loss of rotor rpm experienced by the pilot in this accident. Fuel Control Unit Examination and Testing The engine fuel control unit (FCU) is composed of four main components, namely: the power turbine speed governor, the gas generator speed governor, the acceleration control unit, and the metering unit. The FCU was inspected, bench-tested, disassembled, and examined by Turbomeca in Montral, Quebec, under the direct supervision of a TSB technical investigator. In summary - with the exception of the one minor anomaly noted in the next paragraph - no significant defect was found, and considering the acceptable performance during the engine test-cell runs earlier, no mechanical reason was found that would have caused the FCU to malfunction during the accident. The portion of the FCU bench test that assessed the acceleration of the FCU from idle engine rpm (low fuel flow) to high rpm (high fuel flow) revealed a slight decrement in FCU acceleration characteristics. Information and analysis from Turbomeca reveals that, in the circumstances of this accident, the effect of such a decrement in FCU acceleration would have been unnoticeable to the pilot. In this accident, the engine rpm was already operating at high hover-power values - significantly greater than the idle rpm value used in the bench testing - and the delay in acceleration to full power, if any, would have been negligible and would not have precipitated the loss of rotor rpm experienced by the pilot. The physical position of the throttle cable at the time of the accident had been preserved by the distortion of the engine/transmission deck at impact, kinking the cable at the engine fitting end. This position was recorded and examined in situ, and it was determined that the cable was immoveable as a result of the kinking. Furthermore, the position of the end fitting was consistent with a fully-opened throttle lever, despite the distortion of the engine deck. The TSB Engineering Branch examined the kinked cable fitting to determine if it had been forcibly moved while bent. (Reference: LP098/04) Witness marks on the cable and fitting show that the cable end had moved only 0.055inch while bent, indicating that at impact the cable was trapped very nearly in the position that it had been in during hover flight. During the pilot's attempts to shut down the engine after the rollover, he repeatedly tried to retard the throttle to the stop position. However, the throttle cable was jammed and the lever could not be moved. Subsequently, in his efforts to move the throttle lever, it broke at the cockpit quadrant. He shut the engine down by closing the fuel shut-off control lever located in the same quadrant. Post-crash examinations of the throttle quadrant control tubes, linkages, and rigging revealed no other indication of malfunction or disconnect. Engine Performance Assessment The RFM prescribes that the normal main rotor speed for flight in this helicopter is between 385and 394rpm, and the low rotor warning horn sounds continuously when the rpm falls below360. Before the pilot landed at the accident site, he had carried out an engine Ng difference indicator check using theNg indicator gauge on the instrument panel. The result of this check was 101.4%rpm (Ng) for the prevailing ambient temperature and pressure altitude, which, according to Section4, Normal Procedures of the RFM, was correct for those conditions. Once in the hover on the second take-off attempt, approximate values were as follows: Ng96%, Nr stable at 390rpm, T4temperature 720C, and torque90%. These parameters are consistent with normal, high-power engine performance. At the beginning of the hover-climb and left turn, the Ng and torque each increased past97%. The RFM prescribes that, for this flight, the maximum take-off ratings for Ng, torque, and T4 were 101.4%,100%, and 845C respectively. In-flight testing on a similar Astar helicopter revealed that these high-power engine performance values were achieved only with the throttle lever in the cockpit floor quadrant placed in the expected flight position at lift-off. In contrast, with the throttle placed outside this normal position, the helicopter did not hover with stable rotor rpm nor did it achieve the engine performance readings that the incident pilot observed and accepted before continuing the vertical climb and left turn. Accordingly, considering the physical FCU evidence and the exemplar in-flight testing, it appears that the throttle lever was placed in the appropriate and normally open (flight) position on the accident flight. Accident Site Description The general terrain at the accident site is a 30-degree slope, with the landing pad surrounded by tall conifers. The topography is conducive to down-flowing air at the time of day the accident took place. The temporary helipad had been constructed on a less steep area of the mountainside, and several trees had been felled to provide an unobstructed flight path into and out of the pad site. For all practical purposes, only one approach and departure path could be used; in this case, the approach was northerly, into the prevailing wind. A heel log had been positioned at the back edge of the touchdown area to provide rigid support for the heels of the landing skids when the helicopter landed facing up slope. Helicopter Handling Techniques A notable characteristic of conventional helicopters is that the application of left or right anti-torque pedal by the pilot will affect the total power required to hover.5 In this Astar helicopter, in which the rotor blades turn clockwise when viewed from above, using right pedal to turn the helicopter to the right increases the power demand on the rotor system and can lead to insufficient power to maintain height in the hover, especially in high power-required situations. If full engine power is being used to hover, any application of right pedal demands more total power than is available and degenerates main rotor rpm and lift, leading to hover height loss. A common handling technique to ameliorate this characteristic during situations of high power demand is to turn the helicopter in a direction that requires greater total power - that is, to the right for the Astar - with the benefit that stopping the turn requires less power. In this event, a pilot can frequently recover from a situation of insufficient power and return to the previous hover condition. By contrast, in turning left with the torque then trying to stop the turn by using right tail rotor input, the demand for power by the tail rotor to stop the turn, combined with the power required by the main rotor to maintain the hover height, increases the total power required. If the helicopter is already operating at full power to hover, applying right pedal in the Astar can demand more power than is available, resulting in a loss of rotor rpm and a descent, likely in an overpitched condition.